JP7207558B2 - Gas absorption spectrometry device, frequency locking method, and gas absorption spectrometry method - Google Patents

Description

The present invention relates to a gas absorption spectrophotometer and a gas absorption spectrophotometry method for measuring the concentration of components in a gas to be measured using absorption of laser light.

Laser absorption spectroscopy is widely used as a technique for measuring the concentration of a specific component in a gas to be measured. There are several techniques for laser absorption spectroscopy, one of which is Cavity Ring-Down absorption Spectroscopy (hereinafter sometimes referred to as "CRDS" in common usage). . CRDS is a technique for improving detection sensitivity by lengthening the effective optical path length for light absorption using an optical resonator (see Non-Patent Document 1, etc.).

FIG. 5 is a schematic configuration diagram of a general CRDS device. In FIG. 5, a laser beam having a predetermined wavelength emitted from a laser light source unit 100 is introduced through an optical switch 101 into a tubular measuring cell 20 containing a gas to be measured. At both ends of the measuring cell 20, mirrors 21, 22 with high reflectivity (only a very small amount of light is transmitted) are arranged opposite each other. The measuring cell 20 and the pair of mirrors 21 and 22 constitute the optical resonator 2 . This optical resonator 2 is similar to a Fabry-Perot resonator generally used in, for example, a laser device, and the wavelength (frequency) of light that can resonate is determined according to resonance conditions such as cell length. there is The optical resonator 2 may be a ring-type resonator composed of three or more mirrors instead of a resonator composed of two mirrors facing each other.

A frequency at which the optical resonator 2 can resonate is generally called a mode frequency. FIG. 6 is a spectrum showing the relationship between the mode frequency and the frequency of laser light. As shown in FIG. 6, mode frequencies exist at predetermined frequency intervals. not stored in On the other hand, when the oscillation frequency of the laser light in the laser light source unit 100 is adjusted to match the mode frequency, light power is accumulated in the optical resonator 2 .

In the CRDS device, the optical switch 101 abruptly cuts off the laser light entering the optical resonator 2 while the optical power is sufficiently accumulated in the optical resonator 2 . Then, the light accumulated in the optical resonator 2 just before that travels back and forth between the pair of mirrors 21 and 22 many times (actually, thousands to tens of thousands of times). During this time, the light is gradually attenuated by absorption by the components in the gas to be measured enclosed in the measurement cell 20 . At that time, the photodetector 3 continuously detects the state of attenuation of part of the light that leaks out through the exit-side mirror 22 of the optical resonator 2 . By obtaining the time constant of light attenuation (ringdown time) based on the data detected by the photodetector 3, it is possible to calculate the absorption coefficient of the target component in the gas under measurement at the frequency of the laser light at that time. can. Then, the absolute concentration of the target component can be obtained based on the absorption coefficient. In addition, while scanning the oscillation frequency of the laser light in the laser light source unit 100 within a predetermined range, the measurement of the cavity ring-down (CRD) signal is repeated in the same manner to acquire the absorption spectrum of the target component in the gas to be measured. can also

The following equation (1) is usually used to obtain the absorption coefficient α of the target component in the gas to be measured (see Patent Document 1, etc.).
α=(1/c) {(1/τ)−(1/τ ₀ )} …(1)
Here, c is the speed of light, τ is the ring-down time when the measurement cell 20 contains the gas to be measured, and τ ₀ is the measurement cell 20 when the gas to be measured is not contained (for example, in a vacuum state). ) or when the absorption by the components in the gas to be measured is completely negligible. On the other hand, the relationship between the absorption coefficient α, the number density n, and the absorption cross section σ of the target component (absorbing substance) is given by the following equation (2).
α=nσ (2)

Therefore, using equations (1) and (2), absolute concentrations for components with known absorption cross sections can be calculated from the two ring-down times τ, τ ₀ . In the CRDS apparatus, the optical resonator 2 is used to extend the effective distance (optical path length) for light to pass through the measured gas, so the difference between the two ring-down times τ and τ ₀ becomes large. As a result, it is possible to detect even a very small amount of light absorption by a very small amount of the target component, and it is possible to achieve a higher detection sensitivity than other types of laser absorption spectroscopy.

JP 2011-119541 A Japanese Patent Application Laid-Open No. 2015-32700

Koji Hashiguchi, "Investigative research on highly efficient measurement technology for trace moisture in gas", National Institute of Advanced Industrial Science and Technology, AIST Measurement Standard Report, October 2015, Vol.9, No.2, pp.185-205 R. W. P. Drever, and six others, "Laser Phase and Frequency Stabilization Using an Optical Resonator," Appl. Phys .) B, 1983, Volume 31, Issue 2, pp.97-105

As described above, in the CRDS apparatus, in principle, it is necessary to perform measurement in a state in which the mode frequency of the optical resonator matches the oscillation frequency of the laser light. As a technique for matching the mode frequency of the optical resonator and the oscillation frequency of the laser light, the PDH (Pound-Drever-Hall) method disclosed in detail in Non-Patent Document 2 is often used.

In a CRDS device using the PDH method (see Patent Document 2, etc.), the oscillation frequency of laser light from a laser light source is modulated by a phase modulation element and made incident on an optical resonator. Then, the incident light is reflected back by the optical resonator and is detected by a photodetector, and the detected signal is synchronously detected with the modulated signal to obtain an error signal. This error signal contains information on the difference between the oscillation frequency of the laser light and the mode frequency of the optical resonator, and the error signal becomes smaller as the matching between the oscillation frequency and the mode frequency becomes higher. Therefore, by controlling the oscillation frequency of the laser light in the laser light source section so that the error signal is minimized, the oscillation frequency is matched with the mode frequency. Such feedback control allows the oscillation frequency of the laser light to be locked to the mode frequency of the optical resonator.

If the method of locking the frequency by the PDH method or the like is not used, the CRD signal is acquired after waiting for the timing at which the oscillation frequency of the laser light coincides with the mode frequency of the optical resonator while changing the oscillation frequency of the laser beam, for example. There is a need. Therefore, the timing at which measurements can be performed is quite limited. On the other hand, when the PDH method is used, the oscillation frequency of the laser light can always be matched with the mode frequency, so the number of measurements per unit time can be increased, and the measurement efficiency can be greatly improved. .

However, the conventional CRDS device using the PDH method has the following problems.
In order to feedback-control the oscillation frequency of laser light by the PDH method, an error signal must be obtained at the start of the control. In a general circuit that implements the PDH method, the error signal is generated when the difference between the oscillation frequency of the laser light and the mode frequency is small (generally when it is less than the pulse line width of the longitudinal mode on the frequency spectrum). can only be obtained. On the other hand, in order to achieve high detection accuracy and sensitivity in the CRDS device, it is necessary to narrow the pulse line width of the longitudinal mode (strictly speaking, the resonance condition is such that the pulse line width of the longitudinal mode is narrowed). The frequency range in which the error signal in can be generated is also very narrow. Therefore, when the PDH method is applied to the CRDS device, it is necessary to control the oscillation frequency of the laser light with high accuracy so that the oscillation frequency of the laser light falls within the frequency range of the pulse line width of the very narrow longitudinal mode. There is

In other words, if the pulse line width of the longitudinal mode of the optical resonator is narrowed in order to improve the measurement accuracy and sensitivity of the CRDS device, it becomes necessary to control the oscillation frequency of the laser light with high accuracy, and the laser light source and laser control circuit become necessary. Cost will be higher. Conversely, in order to relax the accuracy of controlling the oscillation frequency of the laser light, it is necessary to widen the pulse line width of the longitudinal mode of the optical resonator. be.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a CRDS apparatus using the PDH method in which laser light is gas absorption spectroscopy measurement device and gas absorption spectroscopy measurement method capable of quickly matching the oscillation frequency of the optical resonator and the mode frequency of the optical resonator, and performing measurement with high accuracy and sensitivity while maintaining the matching state of the frequencies is to provide

One aspect of the gas absorption spectrometry apparatus according to the present invention is an optical resonator that resonates the light emitted from the laser irradiation unit, which includes a laser irradiation unit, a measurement cell containing a gas to be measured, and the light a first detection unit for detecting light extracted from the resonator, the gas absorption spectroscopy measuring device for determining the component concentration in the gas under measurement by cavity ring-down absorption spectroscopy, wherein the laser irradiation unit teeth,
a laser light source whose oscillation frequency can be adjusted;
a frequency conversion unit capable of selectively emitting the laser light emitted from the laser light source with its frequency unchanged or after multiplying the frequency by a predetermined frequency;
a frequency modulation unit that frequency-modulates the laser light emitted from the frequency conversion unit using a modulation signal;
a second detection unit that detects return light from the optical resonator, which is emitted from the optical resonator and is modulated by the frequency modulation unit;
generating an error signal reflecting the difference between the frequency of the laser light irradiated to the optical resonator and the mode frequency of the optical resonator, based on the signal detected by the second detector and the modulated signal; a feedback control unit that controls the oscillation frequency of the laser light source according to the error signal;
Prepare.

Another aspect of the gas absorption spectrophotometer according to the present invention is an optical resonator that resonates light emitted from the laser irradiation unit, the optical resonator including a laser irradiation unit and a measurement cell containing a gas to be measured. a first detection unit for detecting light extracted from the optical resonator, the gas absorption spectroscopy measuring device for determining the component concentration in the gas to be measured by cavity ring-down absorption spectroscopy, wherein the laser irradiation Department is
a laser light source whose oscillation frequency can be adjusted;
a branching section for branching the laser light emitted from the laser light source into a plurality of parts;
a frequency conversion unit that multiplies the frequency of one laser beam branched by the branching unit by a predetermined frequency and emits the same;
a first frequency modulation section that frequency-modulates the laser light emitted from the frequency conversion section using a first modulation signal;
a second frequency modulation section that frequency-modulates the other laser light branched by the branching section using a second modulation signal;
The first laser light modulated by the first frequency modulation section and the second laser light modulated by the second frequency modulation section are optically separated from each other in the optical resonator. a coalescing portion to be incident;
Return light from the optical resonator with respect to the first laser beam irradiated to the optical resonator and return light from the optical resonator with respect to the second laser beam irradiated to the optical resonator , a second detection unit that detects each independently;
generating a first error signal reflecting a difference between the frequency of the first laser light and the mode frequency of the optical resonator, based on the signal detected by the second detector and the first modulated signal; feedback-controlling the oscillation frequency of the laser light source according to the first error signal, thereby matching the oscillation frequency with the mode frequency of the optical resonator, and then maintaining the feedback control state; or Based on the signal detected by the second detector and the second modulation signal for the return light derived from the second laser light, the difference between the frequency of the second laser light and the mode frequency of the optical resonator is determined. a feedback control unit that generates a reflected second error signal and performs feedback control according to the second error signal to continue controlling the oscillation frequency of the laser light source;
Prepare.

Further, one aspect of the frequency locking method according to the present invention is a frequency locking method for locking the oscillation frequency of a laser light source to the resonance frequency of an optical resonator,
a first step of converting the frequency of the laser light from the laser light source to an integral multiple of the oscillation frequency;
a second step of modulating the frequency-converted laser light using a modulation signal;
a third step of causing the modulated laser light to enter the optical resonator;
a fourth step of detecting return light from the optical resonator;
a fifth step of generating an error signal according to the difference between the frequency of the laser light incident on the optical resonator and the mode frequency of the optical resonator, based on the detected returned light and the modulated signal;
a sixth step of feeding back the error signal to control the oscillation frequency of the laser light source;
is executed.

In one aspect of the gas absorption spectroscopy measurement method according to the present invention, the frequency locking method of the above aspect is used, and after matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, the feedback control in the sixth step is performed. A seventh step of performing measurement by cavity ring-down absorption spectroscopy using laser light from the laser light source while maintaining
is further executed.

In another aspect of the gas absorption spectroscopy measurement method according to the present invention, the frequency locking method of the above aspect is used, and after matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, The fundamental frequency laser light from the laser light source is used while frequency-modulating the fundamental frequency laser light using the modulation signal and irradiating the optical resonator, and performing feedback control based on the returned light. a seventh step of performing a measurement by cavity ring-down absorption spectroscopy;
is further executed.

In the gas absorption spectroscopy measurement device, frequency locking method, and gas absorption spectroscopy measurement method of each aspect of the present invention, the method of feedback control of the oscillation frequency of the laser light is based on the PDH method described above. In addition, the “predetermined multiple” when converting the frequency of the laser light emitted from the laser light source unit is typically a natural number (positive integer) multiple other than 1, and is usually double. .

In the gas absorption spectroscopy measurement device, the frequency locking method, and the gas absorption spectroscopy measurement method of each of the above aspects of the present invention, the oscillation frequency of the laser light in the laser light source and the mode frequency of the optical resonator, for example, immediately after starting the device, are If they do not match, the oscillation frequency of the laser light source is adjusted by feedback control by the PDH method using not the fundamental frequency of the laser light but a laser light with a frequency that is a predetermined multiple, for example, twice the fundamental frequency. The reflectance of the mirrors used in the optical resonator depends on the frequency of light, and the reflectance of mirrors exhibiting a high reflectance at the fundamental frequency becomes lower than the reflectance at the fundamental frequency at a predetermined multiple of the frequency. For this reason, the pulse width of the longitudinal mode of the optical resonator at a frequency of a predetermined multiple becomes wider than that at the fundamental frequency, and an appropriate error signal can be obtained even if the difference between the frequency of the laser light and the mode frequency is large to some extent. , good feedback control is performed so that the frequency of the laser light matches the mode frequency.

In this way, according to the gas absorption spectroscopy measuring device, the frequency locking method, and the gas absorption spectroscopy measuring method of each of the above aspects of the present invention, the laser can be The oscillation frequency of light and the mode frequency of the optical resonator can be matched rapidly, and CRDS measurement can be performed with high accuracy and sensitivity while maintaining it.

As a result, it is not necessary to control the oscillation frequency of the laser light with high accuracy, and the cost of the apparatus can be reduced.
In addition, since the oscillation frequency of the laser light and the mode frequency of the optical resonator can be quickly matched and maintained, the number of measurements that can be performed per unit time can be increased, and the measurement efficiency can be improved. can.
In addition, since it is possible to control the oscillation frequency of the laser light to match each mode frequency appearing at regular frequency intervals in order and quickly, the measurement time is also reduced when acquiring CRD signals over a wide frequency range. can be shortened.
Furthermore, since it is possible to adopt an ultra-high reflectance mirror having an extremely high reflectance, which was conventionally difficult to apply the control of the oscillation frequency of the laser light by the PDH method, as a mirror of the optical resonator, CRDS can be achieved. Measurement accuracy and sensitivity can be improved more than before.

1 is a configuration diagram of a main part of a CRDS device that is a first embodiment of the present invention; FIG. 4 is a flowchart showing an example of a procedure of processing before starting measurement in the CRDS device of the first embodiment; FIG. 4 is a diagram showing the frequency dependence of an error signal obtained during feedback control of the frequency of laser light in the CRDS device of the first embodiment; FIG. 2 is a block diagram of the essential parts of a CRDS device that is a second embodiment of the present invention; Schematic configuration diagram of a general CRDS device. FIG. 2 is a schematic spectrum diagram showing the relationship between the mode frequency of an optical resonator and the oscillation frequency of laser light;

[First embodiment]
A first embodiment of a gas absorption spectrophotometer according to the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of the main part of the CRDS device of the first embodiment. FIG. 2 is a flow chart showing an example of the procedure of processing before starting measurement in the CRDS device of this embodiment. FIG. 3 is a diagram showing the frequency dependency of an error signal obtained during feedback control of the frequency of laser light in the CRDS device of this embodiment .

As shown in FIG. 1 , the CRDS device of this embodiment includes a laser irradiation section 1 including a laser light source section 10 , an optical resonator 2 and a main photodetector 3 . The optical resonator 2 and main photodetector 3 are the same as the components in the conventional general CRDS device shown in FIG. 5, and are given the same reference numerals.

In addition to the laser light source unit 10, the laser irradiation unit 1 includes a laser control unit 11, a frequency conversion unit 12, an oscillation unit 13, a phase modulation unit 14, an optical switch 15, a polarization beam splitter 16, a quarter wave plate 17, and secondary light. A detector 18 and a signal processor 19 are included.

The frequency converter 12 selectively emits the laser light of the fundamental frequency (wavelength) ν generated by the laser light source 10 as it is or after converting it into laser light of double the frequency (2ν). . This switching of the frequency of the emitted laser light is mechanically or electrically controlled.
As a specific example, the frequency converter 12 uses an element capable of switching the optical path at high speed, such as a galvanomirror, and switches between an optical path that passes through a nonlinear optical crystal or a quasi-phase matching element and an optical path that does not. , light is incident on both optical paths in parallel, and one of the optical paths is selectively cut off using an acousto-optic modulator (AOM) or the like, thereby switching the frequency of the obtained laser light. be able to. As another example, when using an element such as a quasi-phase matching element whose allowable range of incident wavelengths that can be converted is very narrow, the wavelength conversion efficiency can be greatly changed by the temperature change of the element. It is also possible to switch between the fundamental wave and the harmonic wave.

The phase modulation unit 14 is, for example, an electro-optic modulator (EOM) capable of electrically changing the refractive index of light. It modulates the frequency and phase of the laser light incident through the converter 12 .

The optical switch 15 is the same as the optical switch 101 in FIG. 5, and is an optical element that cuts off the laser beam that has entered through the phase modulation section 14 in a short period of time. The polarizing beam splitter 16 selectively transmits one polarized component (for example, S-polarized light) of the light incident through the optical switch 15, and transmits the incident light that has returned through the quarter-wave plate 17. A return light having a polarization component (for example, P-polarized light) different from that is reflected by an internal reflecting surface in a substantially perpendicular direction.

The quarter-wave plate 17 is an optical element that reversibly changes the state of linear polarization and the state of circular polarization. , changes the polarization state of the light returning from the optical resonator 2 again and returns it to the polarization beam splitter 16 . The secondary photodetector 18 is installed at a position where the return light reflected by the polarization beam splitter 16 can be detected, and outputs an electrical signal corresponding to the intensity of the return light. The signal processing unit 19 includes an error signal generation unit 191 and a lock determination unit 192 as functional blocks. The laser control section 11 adjusts the oscillation frequency of the laser light source section 10 according to the signal from the signal processing section 19 .

The operation of the CRDS device of this embodiment will be described.
Here, as an example, it is assumed that the reciprocating cavity length L of the optical cavity 2 is 1 m, and the reflectance R of the mirrors 21 and 22 with respect to the fundamental frequency ν of the laser light when the frequency conversion is not performed in the frequency converter 12 is 99.0. Assume that the reflectance R of the mirrors 21 and 22 is 99%, and that the reflectance R of the mirrors 21 and 22 is 99% when the laser light of frequency 2ν obtained by performing frequency conversion in the frequency converter 12 is incident on the optical resonator 2 . The pulse linewidth of the longitudinal mode in the optical resonator 2 can be expressed by the following equation (3).
ν _FWHM ≈ {c(1−R)}/2πL√R (3)
Equation (3) means that the reflectance of the mirrors 21 and 22 has a large effect on the longitudinal mode pulse line width.

According to the equation (3), the longitudinal mode pulse full width at half maximum at frequency ν is approximately 5 kHz, and the longitudinal mode pulse full width at half maximum at frequency 2ν is approximately 0.5 MHz (=500 kHz). Assuming that the frequency range in which the error signal can be properly obtained in the PDH method is about twice the full width at half maximum of the longitudinal mode pulse, the frequency range in which the error signal can be obtained is ±5 kHz at frequency ν and 2ν at frequency 2ν. is ±0.5 MHz. That is, by doubling the frequency of the laser light introduced into the optical resonator 2, the frequency range in which the error signal can be obtained is expanded approximately 100 times, and the accuracy of the initial laser light frequency is increased. greatly alleviated.

In FIG. 3, the solid line shows the frequency dependence of the error signal generated by the error signal generator 191 when light of the fundamental frequency ν is used. When the oscillation frequency of the laser light is controlled so that the absolute value of the error signal becomes small (close to zero), an appropriate error signal can be obtained only in the frequency range where the error signal changes monotonically with respect to the frequency. As shown in FIG. 3, the frequency range over which a suitable error signal can be obtained for light of fundamental frequency ν is very narrow. In contrast, the error signal for light with a fundamental frequency of 2ν, as indicated by the dashed line, considerably extends the frequency range over which a suitable error signal can be obtained. If the original frequency of the laser light falls within this frequency range, the PDH method adjusts the frequency of the laser light so that it matches the mode frequency. Therefore, in the CRDS apparatus of this embodiment, the frequency of laser light is adjusted in the following procedure when performing CRDS measurement.

In an initial state in which the fundamental frequency of the laser light does not match the mode frequency of the optical resonator 2, the lock determination unit 192 outputs a signal indicating the unlocked state. Upon receiving this signal, the frequency conversion unit 12 doubles the frequency of the laser light incident from the laser light source unit 10 and emits laser light with a frequency of 2ν (step S1).

This laser light is frequency-modulated by the phase modulating section 14 based on the modulated signal generated by the oscillating section 13, passes through the optical switch 15, the polarization beam splitter 16, and the quarter-wave plate 17, and irradiates the optical resonator 2. be done. A particular polarization component in a linear polarization state (eg, S-polarization) is selected when passing through the polarizing beam splitter 16 , and the linear polarization is converted to circular polarization when passing through the quarter-wave plate 17 . The laser light irradiated to the optical resonator 2 enters the measuring cell 20 and is reflected back and forth between the pair of mirrors 21 and 22 . .

This return light passes through the quarter-wave plate 17 and reaches the polarization beam splitter 16 . When circularly polarized light is converted into linearly polarized light by the quarter-wave plate 17 , it becomes a different polarized component (for example, P-polarized light) from the time of previous incidence. and head to the secondary photodetector 18 . The secondary photodetector 18 outputs a detection signal corresponding to the intensity of the incident return light. A detection signal based on the returned light contains information about the resonance mode of the optical resonator 2 . The error signal generator 191 generates an error signal containing information on the difference between the mode frequency of the optical resonator 2 and the frequency 2ν of the laser light from this detection signal and the modulated signal obtained from the oscillator 13 . The smaller the difference between the frequency 2ν of the laser light and the mode frequency, the smaller the error signal (absolute value). The laser control section 11 controls the oscillation frequency of the laser light in the laser light source section 10 so that the error signal is reduced. That is, by executing feedback control by the PDH method, the frequency 2ν of the laser light and the mode frequency in the optical resonator 2 are matched (step S2).

When the fundamental frequency ν of the laser light matches the mode frequency and the laser light resonates in the optical resonator 2 , the laser light with the double frequency 2ν also resonates in the optical resonator 2 . . This is because the resonance condition of the optical resonator 2 is as follows, where f is the resonance frequency and L is the resonator length of the round trip:
f=n・(c/L) (where n is a natural number) …(4)
Therefore, when the light of the fundamental frequency resonates, naturally the light of the integral multiple of that frequency also resonates.

As described above, when the frequency of the laser light introduced into the optical resonator 2 is 2ν, the frequency range over which the error signal can be obtained is as wide as ±0.5 MHz. Therefore, even if the difference between the mode frequency and the frequency twice the initial oscillation frequency ν of the laser light in the laser light source unit 10 is large to some extent, that is, even if the oscillation frequency is not controlled with high accuracy, the appropriate It is possible to generate an error signal necessary for effective feedback control and perform frequency locking operation.

The error signal generated by the error signal generator 191 reflects the difference between the frequency 2ν of the laser light and the mode frequency in real time. Therefore, the lock determination unit 192 repeatedly determines whether the deviation between the frequency 2ν and the mode frequency is within a predetermined frequency range based on the generated error signal (step S3). Specifically, it may be determined whether or not the frequency ν falls within a frequency range in which the error signal can be obtained, for example, the mode frequency ±5 kHz. The lock determination unit 192 outputs a lock detection signal indicating a locked state when the frequency deviation falls within a predetermined frequency range (Yes in step S3).

According to the lock detection signal, the frequency converter 12 switches the frequency of the emitted laser light from 2ν to ν, that is, the fundamental frequency (step S4). The resonance modes of the optical resonator 2 appear at equal frequency intervals from the frequency 0 on the frequency axis. Therefore, if the frequency of the laser light and the mode frequency are sufficiently matched at the frequency 2ν of the laser light, the frequency and the mode frequency are sufficiently matched even if the frequency of the laser light is changed to ν. Therefore, even after the frequency of the laser light is switched in the frequency conversion unit 12, the feedback control by the PHD method is continued in the same manner as before, and the oscillation frequency of the laser light in the laser light source unit 10 is sufficiently equal to the mode frequency. The matched state is maintained (step S5). Of course, at this time, the lock determination section 192 continues to output a lock detection signal indicating that the frequency deviation is within a predetermined frequency range.

Then, while the frequency of the laser light introduced into the optical resonator 2 is maintained at ν, measurement by CRDS is performed (step S6). That is, after the power of laser light is accumulated inside the optical resonator 2 , the light is blocked by the optical switch 15 , and the main photodetector 3 detects the intensity change of the light leaking through the exit side mirror 22 . At this time, since the pulse line width of the longitudinal mode is narrow and the reflectance of the mirrors 21 and 22 is very high, highly sensitive measurement can be performed. In addition, since the PDH method keeps the oscillation frequency of the laser light consistent with the mode frequency, the number of CRDS measurements per unit time can be increased, and high measurement efficiency can be achieved.

As described above, in the CRDS apparatus of the present embodiment, highly sensitive CRDS measurement can be performed without controlling the oscillation frequency of the laser light in the laser light source unit 10 with high accuracy. In addition, since the oscillation frequency of the laser light and the mode frequency of the optical resonator 2 can be matched quickly, the measurement can be started immediately, and the number of measurements per unit time can be increased, and the measurement efficiency can be improved. .

Also, when it is desired to obtain a CRD signal over a wide frequency range, the CRD signal at each mode frequency is obtained while sequentially matching the oscillation frequency of the laser light to each resonance mode appearing at regular frequency intervals. There is a way to go. In the CRDS apparatus of the present embodiment, the oscillation frequencies of the laser light can be matched in order and quickly with the respective frequencies of a plurality of modes. Beneficial.

In the configuration shown in FIG. 1, the arrangement of each part including the optical element is an example, and the arrangement may be appropriately arranged, such as arranging the optical switch 15 between the quarter-wave plate 17 and the optical resonator 2. It is of course possible to change.

[Second embodiment]
Next, a second embodiment of the gas absorption spectrophotometer according to the present invention will be described with reference to the accompanying drawings.
FIG. 4 is a configuration diagram of the main part of the CRDS device of the second embodiment. In this CRDS device, instead of switching between laser light with a frequency of ν and laser light with a frequency of 2ν to be incident on the optical resonator 2, two laser beams with different frequencies are simultaneously incident on the optical resonator 2. It's becoming

As shown in FIG. 4, the laser irradiation unit 4 includes a laser light source unit 40, an optical coupler 41, a second oscillation unit 42, a second phase modulation unit 43, a second optical circulator 44, a half-wave plate 45, a frequency converter. Section 46, First Oscillating Section 47, First Phase Modulating Section 48, First Optical Circulator 49, Polarizing Beam Splitter 50, Optical Switch 51, First Secondary Photodetector 52, Second Secondary Photodetector 53, Signal Processing Section 54, and a laser controller 55. As in the device of the first embodiment, the signal processing section 54 includes an error signal generation section 541 and a lock determination section 542 as functional blocks.

The optical coupler 41 is an optical element that splits incident light into a plurality of (here, two) light, and may be a splitter. The first and second optical circulators 49 and 44 are non-reciprocating and unidirectional optical elements having three ports. That is, the light incident on the first port is emitted only to the second port, and the light incident on the second port is emitted only to the third port. Also, the half-wave plate 45 is an optical element that rotates the incident linearly polarized light by a predetermined angle and emits it.

The operation of the CRDS device of this embodiment when performing CRDS measurement will be described.
The laser light of frequency ν generated by the laser light source unit 40 is branched into two systems by the optical coupler 41 , one of which is introduced into the second phase modulation unit 43 and the other into the frequency conversion unit 46 through the half-wave plate 45 . be done. The frequency (and phase) of the laser light of frequency ν is modulated by the second phase modulation section 43 based on the modulated signal generated by the second oscillation section 42 , and is incident on the polarization beam splitter 50 through the second optical circulator 44 . be. On the other hand, the half-wave plate 45 rotates the polarization direction of the linearly polarized light of the laser beam by a predetermined angle, thereby converting, for example, S-polarized light into P-polarized light. The frequency conversion unit 46 doubles the frequency of the laser light from ν to 2ν, and the first phase modulation unit 48 converts the frequency of the laser light of the frequency 2ν based on the modulation signal generated by the first oscillation unit 47. modulate the The modulated laser light enters the polarization beam splitter 50 through the first optical circulator 49 .

The polarizing beam splitter 50 selectively transmits a specific polarized component (for example, S-polarized light) of the laser light of frequency ν (which is modulated) incident through the second optical circulator 44 and directs it to the optical switch 51 . Also, the polarizing beam splitter 50 selectively reflects a specific polarized light component (for example, P-polarized light) of the laser light of frequency 2ν (which is modulated) incident through the first optical circulator 49 , and sends it to the optical switch 51 . make it go Therefore, when the optical switch 51 is open, both the S-polarized laser light with a frequency of ν and the P-polarized laser light with a frequency of 2ν are introduced into the optical resonator 2 .

The S-polarized laser light with a frequency ν and the P-polarized laser light with a frequency 2ν are reflected back and forth between the pair of mirrors 21 and 22. Return to splitter 50 . The polarization state of this return light is the same as the polarization state of the original irradiation light. Therefore, the return light, which is S-polarized light with frequency ν, passes through the polarization beam splitter 50 and returns to the second optical circulator 44 , changes its traveling direction by the second optical circulator 44 , and enters the second sub-photodetector 53 . . On the other hand, the return light, which is P-polarized light with a frequency of 2ν, is reflected by the polarization beam splitter 50 and returns to the first optical circulator 49 , changes its traveling direction by the first optical circulator 49 , and enters the first sub-photodetector 52 . .

As described above, both the light of wavelength ν and the light of wavelength 2ν resonate in the optical resonator 2 . Therefore, both the detection signal from the first secondary photodetector 52 and the detection signal from the second secondary photodetector 53 contain information about the resonance mode of the optical resonator 2 . The error signal generator 541 in the signal processor 54 determines the difference between the mode frequency of the optical resonator 2 and the frequency ν of the laser light from the detection signal from the second secondary photodetector 53 and the modulation signal from the second oscillator 42. and the mode frequency of the optical resonator 2 and the frequency 2ν of the laser light from the detection signal from the first secondary photodetector 52 and the modulation signal from the first oscillator 47. A first error signal containing difference information is generated.

In the initial state in which the fundamental frequency of the laser light does not match the mode frequency of the optical resonator 2, the lock determination unit 542 outputs a signal indicating the unlocked state. In this case, the error signal generation unit 541 selects an error signal based on the return light of frequency 2ν and sends it to the laser control unit 55, and the laser control unit 55 controls the laser light in the laser light source unit 40 so that the error signal becomes small. control the oscillation frequency of That is, the oscillation frequency of the laser light and the mode frequency are matched by performing feedback control by the PDH method using the laser light of the frequency 2ν.

The lock determination unit 542 determines from the two error signals whether or not the deviation between the oscillation frequency of the laser light and the mode frequency is within a predetermined frequency range. Outputs a lock detection signal. Upon receiving this signal, the error signal generator 541 selects the error signal based on the return light of the frequency ν instead of the error signal based on the return light of the frequency 2ν and switches the operation so as to send it to the laser control unit 55 . After switching in this way, feedback control is performed by the PHD method using laser light of frequency ν, and the oscillation frequency of the laser light in the laser light source unit 10 is maintained in a state of sufficiently matching the mode frequency. .

In this way, the CRDS measurement is performed in a state where the oscillation frequency of the laser light in the laser light source unit 10 sufficiently matches the mode frequency. Although CRDS can be measured using a laser beam with a frequency of 2ν, the ring-down time is shorter at the frequency of 2ν because the reflectance of the mirrors 21 and 22 is lower than when the frequency is ν. , the measurement accuracy and sensitivity are reduced accordingly. Since there is practically no advantage in measuring CRDS using laser light with a frequency of 2ν, CRDS may be measured using laser light with a frequency of ν.

[Modification of Second Embodiment]
In the CRDS device of the second embodiment, as in the CRDS device of the first embodiment, the frequency of light used for feedback control by the PDH method is switched from the initial 2ν to ν. Generally, once the oscillation frequency and mode frequency of the laser light match, it is better to implement feedback control by the PHD method using light of frequency ν to suppress fluctuations in the oscillation frequency of the laser light and maintain high accuracy. because it can However, even when feedback control is performed by the PHD method using light with a frequency of 2ν, fluctuations in the oscillation frequency of the laser light do not substantially pose a problem in terms of measurement accuracy.

Therefore, in the CRDS apparatus of the second embodiment, the feedback control of the oscillation frequency of the laser light by the PDH method is performed using the light of the frequency 2ν, and the error signal obtained for the light of the frequency ν is measured by the CRDS. The configuration may be changed so that it is used only for determining that it has become possible. Specifically, the error signal generator 541 always uses the light of frequency 2ν as the error signal to be sent to the laser controller 55, and the lock determination unit 542 converts the error signal obtained for the light of frequency ν into a predetermined error signal. When it falls within the range, a lock detection signal is output, and the control unit (not shown) measures the CRDS after the lock detection signal is obtained. Even with such a configuration, similarly to the CRDS devices of the first and second embodiments, highly accurate and highly sensitive measurement can be performed without highly accurate control of the oscillation frequency of the laser light.

Although the oscillation frequency (fundamental frequency) of the laser light is doubled in the above embodiment, it may be multiplied by a natural number other than two. It is clear from the above description that resonance occurs in the optical resonator 2 even when the frequency of the laser light is changed in this way. In practice, however, the frequency range over which a suitable error signal can be obtained is greatly expanded by simply doubling the fundamental frequency, so there is practically no need to multiply the fundamental frequency by a factor of 3 or more. .

In addition, since the frequency of the light resonating in the optical resonator 2 depends on the length of the reciprocating resonator, in the device of the first embodiment, when the oscillation frequency of the laser light is multiplied by n, the n is a natural number. Even if there is a deviation, resonance can be generated by finely adjusting the reciprocating cavity length of the optical cavity 2 . Therefore, it is not necessary to strictly multiply the oscillation frequency of the laser light by a natural number.

In addition, all of the above-described embodiments are examples of each aspect of the present invention, and it is obvious that even if deformation, modification, addition, etc. is.

[Various aspects]
Although the embodiments of the present invention have been described above with reference to the drawings, those skilled in the art will easily understand that the above-described embodiments and modifications thereof are specific examples of the following aspects.

(Section 1) A gas absorption spectrophotometer according to an aspect of the present invention includes a laser irradiation unit and a measurement cell containing a gas to be measured. and a first detector for detecting light extracted from the optical resonator, the gas absorption spectroscopy measuring device for determining the concentration of components in the gas to be measured by cavity ring-down absorption spectroscopy, , the laser irradiation unit,
a laser light source whose oscillation frequency can be adjusted;
a frequency conversion unit capable of selectively emitting the laser light emitted from the laser light source with its frequency unchanged or after multiplying the frequency by a predetermined frequency;
a frequency modulation unit that frequency-modulates the laser light emitted from the frequency conversion unit using a modulation signal;
a second detection unit that detects return light from the optical resonator, which is emitted from the optical resonator and is modulated by the frequency modulation unit;
generating an error signal reflecting the difference between the frequency of the laser light irradiated to the optical resonator and the mode frequency of the optical resonator, based on the signal detected by the second detector and the modulated signal; a feedback control unit that controls the oscillation frequency of the laser light source according to the error signal;
is provided.

In the gas absorption spectroscopy measurement device according to item 1, when the oscillation frequency of the laser light in the laser light source and the mode frequency of the optical resonator do not match, such as immediately after starting the device, the fundamental frequency of the laser light The oscillation frequency of the laser light source is adjusted so as to match the mode frequency by feedback control by the PDH method using not the frequency but the laser light of the frequency multiplied by a predetermined frequency.

Since the pulse width of the longitudinal mode of the optical resonator at a predetermined multiple frequency is wider than that at the fundamental frequency, the frequency range in which an error signal suitable for feedback control by the PDH method can be obtained is widened. As a result, even if the difference between the frequency of the laser light and the mode frequency is large to some extent, good feedback control is performed so that the frequency of the laser light matches the mode frequency, and the oscillation frequency of the laser light is adjusted. be. Then, when the oscillation frequency of the laser light sufficiently matches the mode frequency, the process shifts to feedback control by the PDH method using the laser light of the fundamental frequency. The measurement is performed in a state consistent with .

In this way, according to the gas absorption spectrophotometer described in item 1, the oscillation frequency of the laser light and the mode frequency of the optical resonator can be adjusted without controlling the oscillation frequency of the laser light in the laser light source with high accuracy. Rapid agreement and maintenance of high accuracy and sensitivity CRDS measurements can be performed. Since there is no need to control the oscillation frequency of the laser light with high precision, the cost of the device can be reduced. In addition, since the oscillation frequency of the laser light and the mode frequency of the optical resonator can be quickly matched and maintained, the number of measurements that can be performed per unit time can be increased, and the measurement efficiency can be improved. can.

(Section 2) A gas absorption spectrophotometer according to another aspect of the present invention includes a laser irradiation unit and a measurement cell containing a gas to be measured. A gas absorption spectroscopy measuring apparatus comprising a resonator and a first detection section for detecting light extracted from the optical resonator, and determining the concentration of components in the gas to be measured by cavity ring-down absorption spectroscopy. and the laser irradiation unit
a laser light source whose oscillation frequency can be adjusted;
a branching section for branching the laser light emitted from the laser light source into a plurality of parts;
a frequency conversion unit that multiplies the frequency of one laser beam branched by the branching unit by a predetermined frequency and emits the same;
a first frequency modulation section that frequency-modulates the laser light emitted from the frequency conversion section using a first modulation signal;
a second frequency modulation section that frequency-modulates the other laser light branched by the branching section using a second modulation signal;
The first laser light modulated by the first frequency modulation section and the second laser light modulated by the second frequency modulation section are optically separated into the optical resonator together. a coalescing portion to be incident;
Return light from the optical resonator with respect to the first laser beam irradiated to the optical resonator and return light from the optical resonator with respect to the second laser beam irradiated to the optical resonator , a second detection unit that detects each independently;
generating a first error signal reflecting a difference between the frequency of the first laser light and the mode frequency of the optical resonator based on the signal detected by the second detector and the first modulated signal; feedback-controlling the oscillation frequency of the laser light source according to the first error signal, thereby matching the oscillation frequency with the mode frequency of the optical resonator, and then maintaining the feedback control state; or a difference between the frequency of the second laser light and the mode frequency of the optical resonator based on the detection signal by the second detector and the second modulation signal for the return light originating from the second laser light; a feedback control unit that generates a reflected second error signal and performs feedback control according to the second error signal to continue controlling the oscillation frequency of the laser light source;
is provided.

In the gas absorption spectroscopy measuring apparatus according to the above item 1, the frequency of the laser light is switched in the loop for feedback-controlling the oscillation frequency of the laser light in the laser light source, whereas the gas absorption spectroscopy according to the above item 2. In the measuring device, the error signal used for controlling the oscillation frequency of the laser light in the laser light source is switched in the loop for feedback-controlling the oscillation frequency of the laser light in the laser light source. In other words, electrical signals are switched instead of light. Therefore, even in the gas absorption spectrophotometer described in item 2, when the oscillation frequency of the laser light in the laser light source and the mode frequency of the optical resonator do not match, such as immediately after starting the device, the fundamental frequency is The oscillation frequency of the laser light source is adjusted to match the mode frequency by feedback control by the PDH method using a laser beam with a frequency multiplied by a predetermined frequency. Then, when the oscillation frequency of the laser light sufficiently matches the mode frequency, the process shifts to feedback control by the PDH method using the laser light of the fundamental frequency. Measurements are performed in accordance with

According to the gas absorption spectroscopy measurement device described in item 2, similarly to the device described in item 1, without controlling the oscillation frequency of the laser light in the laser light source with high accuracy, the oscillation frequency of the laser light and the light The mode frequencies of the resonator can be rapidly matched and maintained with high accuracy and sensitivity for CRDS measurements. Since there is no need to control the oscillation frequency of the laser light with high precision, the cost of the device can be reduced. In addition, since the oscillation frequency of the laser light and the mode frequency of the optical resonator can be quickly matched and maintained, the number of measurements that can be performed per unit time can be increased, and the measurement efficiency can be improved. can.

(Section 3) A frequency locking method according to an aspect of the present invention is a frequency locking method for locking the oscillation frequency of a laser light source to the resonance frequency of an optical resonator,
a first step of converting the frequency of the laser light from the laser light source to an integral multiple of the oscillation frequency;
a second step of modulating the frequency-converted laser light using a modulation signal;
a third step of causing the modulated laser light to enter the optical resonator;
a fourth step of detecting return light from the optical resonator;
a fifth step of generating an error signal according to the difference between the frequency of the laser light incident on the optical resonator and the mode frequency of the optical resonator, based on the detected returned light and the modulated signal;
a sixth step of feeding back the error signal to control the oscillation frequency of the laser light source;
is executed.

According to the frequency locking method according to item 3, the oscillation frequency of the laser light and the mode frequency of the optical resonator are quickly matched without controlling the oscillation frequency of the laser light in the laser light source with high precision, and frequency can be locked.

(Section 4) A gas absorption spectroscopy measurement method according to an aspect of the present invention uses the frequency locking method described in Section 3,
After matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, while maintaining the feedback control in the sixth step, measurement by cavity ring-down absorption spectroscopy using the laser light from the laser light source. a seventh step to perform,
is further executed.

(Section 5) A gas absorption spectroscopy measurement method according to another aspect of the present invention uses the frequency locking method described in Section 3,
After matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, the fundamental frequency laser light emitted from the laser light source is frequency-modulated using a modulation signal and irradiated to the optical resonator; A seventh step of performing measurement by cavity ring-down absorption spectroscopy using the fundamental frequency laser light from the laser light source while performing feedback control based on the returned light;
is further executed.

According to the gas absorption spectroscopy measurement method described in paragraphs 4 and 5, measurement is performed in a state where the oscillation frequency of the laser light sufficiently matches the mode frequency, so CRDS measurement with high accuracy and sensitivity is performed. can do.

DESCRIPTION OF SYMBOLS 1, 4... Laser irradiation part 10, 40... Laser light source part 11, 55... Laser control part 12, 46... Frequency conversion part 13, 42, 47... Oscillation part 14, 43, 48... Phase modulation part 15, 51... Light Switches 16, 50 Polarizing beam splitter 17 Quarter wave plate 18, 52, 53 Secondary photodetector 19, 54 Signal processor 191, 541 Error signal generator 192, 542 Lock determination unit 2 Light Resonator 20... Measuring cells 21, 22... Mirror 3... Main photodetector 41... Optical couplers 44, 49... Optical circulator 45... Half-wave plate

Claims (5)

A laser irradiation section, an optical resonator that resonates the light emitted from the laser irradiation section, and a first detection section that detects the light extracted from the optical resonator. and a gas absorption spectrometer for obtaining the concentration of a component in the gas to be measured by cavity ring-down absorption spectroscopy, wherein the laser irradiation unit includes:
a laser light source whose oscillation frequency can be adjusted;
a frequency conversion unit capable of selectively emitting the laser light emitted from the laser light source with its frequency unchanged or after multiplying the frequency by a predetermined frequency;
a frequency modulation unit that frequency-modulates the laser light emitted from the frequency conversion unit using a modulation signal;
a second detection unit that detects return light from the optical resonator, which is emitted from the optical resonator and is modulated by the frequency modulation unit;
generating an error signal reflecting the difference between the frequency of the laser light irradiated to the optical resonator and the mode frequency of the optical resonator, based on the signal detected by the second detector and the modulated signal; a feedback control unit that controls the oscillation frequency of the laser light source according to the error signal;
A gas absorption spectrophotometer. A laser irradiation section, an optical resonator that resonates the light emitted from the laser irradiation section, and a first detection section that detects the light extracted from the optical resonator. and a gas absorption spectrometer for obtaining the concentration of a component in the gas to be measured by cavity ring-down absorption spectroscopy, wherein the laser irradiation unit includes:
a laser light source whose oscillation frequency can be adjusted;
a branching section for branching the laser light emitted from the laser light source into a plurality of parts;
a frequency conversion unit that multiplies the frequency of one laser beam branched by the branching unit by a predetermined frequency and emits the same;
a first frequency modulation section that frequency-modulates the laser light emitted from the frequency conversion section using a first modulation signal;
a second frequency modulation section that frequency-modulates the other laser light branched by the branching section using a second modulation signal;
The first laser light modulated by the first frequency modulation section and the second laser light modulated by the second frequency modulation section are optically separated from each other in the optical resonator. a coalescing portion to be incident;
Return light from the optical resonator with respect to the first laser beam irradiated to the optical resonator and return light from the optical resonator with respect to the second laser beam irradiated to the optical resonator , a second detection unit that detects each independently;
generating a first error signal reflecting a difference between the frequency of the first laser light and the mode frequency of the optical resonator, based on the signal detected by the second detector and the first modulated signal; feedback-controlling the oscillation frequency of the laser light source according to the first error signal, thereby matching the oscillation frequency with the mode frequency of the optical resonator, and then maintaining the feedback control state; or Based on the signal detected by the second detector and the second modulation signal for the return light derived from the second laser light, the difference between the frequency of the second laser light and the mode frequency of the optical resonator is determined. a feedback control unit that generates a reflected second error signal and performs feedback control according to the second error signal to continue controlling the oscillation frequency of the laser light source;
A gas absorption spectrophotometer.
A frequency locking method for locking the oscillation frequency of a laser light source to the resonance frequency of an optical resonator,
a first step of converting the frequency of the laser light to an integral multiple of the oscillation frequency;
a second step of modulating the frequency-converted laser light using a modulation signal;
a third step of causing the modulated laser light to enter the optical resonator;
a fourth step of detecting return light from the optical resonator;
a fifth step of generating an error signal according to the difference between the frequency of the laser light incident on the optical resonator and the mode frequency of the optical resonator, based on the detected returned light and the modulated signal;
a sixth step of feeding back the error signal to control the oscillation frequency of the laser light source;
frequency lock method to perform. Using the frequency locking method of claim 3,
After matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, while maintaining the feedback control in the sixth step, measurement by cavity ring-down absorption spectroscopy using the laser light from the laser light source. a seventh step to perform,
gas absorption spectroscopy method to further perform.
Using the frequency locking method of claim 3,
After matching the oscillation frequency of the laser light source to the mode frequency of the optical resonator, the fundamental frequency laser light emitted from the laser light source is frequency-modulated using a modulation signal and irradiated to the optical resonator; A seventh step of performing measurement by cavity ring-down absorption spectroscopy using the fundamental frequency laser light from the laser light source while performing feedback control based on the returned light;
gas absorption spectroscopy method to further perform.

Images